This invention relates to relative pressure sensors. With such sensors, pressures of media, such as liquids, gases or vapors, can be measured, the measurement being made relative to the current atmospheric or ambient pressure, i.e., this atmospheric or ambient pressure serving as a reference pressure.
U.S. Pat. No. 5,747,694 discloses a relative pressure sensor comprising a sensing element which is fixed to a glass pedestal and which is connected with a medium under pressure by a bore through the glass pedestal serving to guide reference air, and a housing having an interior space, a pressure connection, a lateral chamber accessible to the reference air, said lateral chamber having a hydrophobic, open-pore filter fitted therein in a plane above the sensing element, sealing the lateral, and a reference air chamber located above the sensing element into which the reference flows from the filter.
Further, U.S. Pat. No. 5,079,953 discloses, as one of three variants, a relative pressure sensor comprising a capacitive sensing element having a diaphragm and a substrate which is connected along an edge with an edge of the diaphragm by a pressure-proof joint to form a chamber, and which has a bore extending there through from an outer side to the chamber for guiding reference air, an external surface of the diaphragm coming into direct contact with a medium under pressure in a measurement operation; and a housing having an interior space, a connecting portion which has a pressure connection formed thereon, and in which the diaphragm is fitted, sealing the interior space, a cover, and a bore extending from a front side of the connecting portion through a wall of the housing and the cover to the interior space for guiding reference air.
Usually, and this is not mentioned in U.S. Pat. No. 5,079,953, the outlet opening of the reference-air bore in the connecting portion, i.e., the front portion is closed with an open-pore, hydrophobic polytetrafluoroethylene filter plug to prevent contaminating particles from getting into the interior of the housing, and thus to the sensing element, to repel splashing and/or condensed water, and to prevent water droplets from being sucked into the interior.
In the assembly disclosed in the U.S. Pat. No. 5,079,053, in contrast to the assembly of U.S. Pat. No. 5,747,694, the out let of the reference air bore is located at a lower level than the diaphragm, so that the reference air has already reached the dew point opf moisture contained in it. Thus, water condensed out of this moisture can flow off to the outside before the reference air gets into the interior of he housing.
That is not the case with the assembly of U.S. Pat. No. 5,747,694, so that despite the hydrophobic filter, moisture-saturated air can reach the sensing element. Also, that U.S. Patent gives no information on the material the filter is made of; therefore, the filter will generally be a poly-tetrafluoroethylene filter as mentioned above.
When redesigning the relative pressure sensor according to U.S. Pat. No. 5,079,053 to eliminate the need for the pressure connection and allow the medium to come into direct contact with virtually the entire surface of the diaphragm, i.e., when creating a so-called flush-diaphragm relative pressure sensor, it turned out that the polytetrafluoroethylene filter plug and the associated outlet opening have to be disposed laterally in the front portion of the housing, so that the dew point of the reference air is located not outside the interior of the housing, but practically at the sensing element. Consequently, the water vapor will condense in the interior of the housing and at or in the sensing element, and both this element and then the housing will gradually fill up with water.
This is mainly due to the fact that the polytetrafluoroethylene filter plug has a very low thermal conductivity in comparison with the metal of the housing. Thus, reference air saturated with water vapor, which is warmer than the diaphragm, will leak to the interior of the housing without its moisture condensing out at the polytetrafluoroethylene filter plug, even if the front portion of the housing is at the temperature of the current dew point of the reference air.
lt is therefore an object of the invention to provide a flush-diaphragm relative pressure sensor in which the reference air reaches its current dew point before getting into the interior of the housing.
To attain this object, the invention provides a relative pressure sensor comprising a capacitive, resistive, or piezoelectric sensing element having a diaphragm an external surface of which comes into direct contact with a medium under pressure in a measurement operation; and a housing having an interior space, a front portion substantially open across the interior space, in the vicinity of which the diaphragm closes the interior space such that the front portion projects beyond the diaphragm, and a bore extending through the front portion from an outer side thereof to the interior space of the housing for guiding reference air which is closed from the outer side with an open-pore, highly heat-conducting and hydrophobic filter, so that the filter and the diaphragm are at the same or nearly the same temperature.
In a first preferred embodiment of the invention, the filter is made of a sintered metal rendered hydrophobic or of a metallic sponge rendered hydrophobic; the sintered metal is preferably of high-grade steel or bronze, and the metallic sponge is preferably of made of titanium or zirconium.
In a second preferred embodiment of the invention, a capacitive sensing element is provided which comprises in addition to the diaphragm a substrate which is connected along an edge with an edge of the diaphragm by a pressure-proof joint to form a chamber, and through which extends a bore from an outer side to the chamber for guiding reference air, and which capacitive sensing element has an electrode on an inner side of the diaphragm and at least one electrode on an opposite inner side of the substrate, with electrical contact being made to the electrode(s) of the substrate from the outer side of and through the substrate, and electrical contact being made to the electrode of the diaphragm through the joint.
In a third preferred embodiment of the invention, a resistive sensing element is provided wherein a single strain gage or a half-bridge containing two strain gages or a full bridge containing four strain gages is disposed on the diaphragm.
In a fourth preferred embodiment of the invention, a piezoelectric sensing element is provided wherein at least one piezoelectric device is disposed on the diaphragm.
One advantage of the invention is that because of the high thermal conductivity of the filter, the dew point of the reference air is now always located at the filter itself, because it is ensured that the surface temperature of the filter is virtually equal to the temperature of the diaphragm and the medium.
Therefore, the filter, because of its hydrophobic properties cannot become saturated with water, but the water condensed out of the reference air will be drained off to the outside. The temperature in the interior of the housing and in or at the sensing element does not fall below the dew point, so that no condensed water will form in the interior.